1. Field of the Invention
This invention relates broadly to liquid chromatography methods and devices. More particularly, it concerns methods and devices to introduce substance samples into the chromatography column and especially in micro-scale chromatography procedures.
2. Description of the Prior Art
Conventional methods for introducing a sample into a liquid chromatograph include injection of the sample by a syringe near the inlet of the column or the valve switching of a sample loaded loop into the flow stream ahead of the column. Such methods can limit the chromatographic efficiency of the analysis, especially in micro-scale liquid chromatography. For example, syringe introduction of the sample into the solvent filled space ahead of the column causes dispersion of the sample into the neighboring solvent and causes spreading of the chromatographic peaks. On the other hand, syringe introduction into the column bed results in a non-uniform concentration profile and frequent plugging of the syringe needle with resulting peak spreading and nonrepeatable quantity of introduced sample.
With valve switching of a sample loaded loop into the flow stream, this inevitably results in dispersion of the sample in the solvent and broadened chromatogrpahic peaks. This dispersion results from changes in flow diameters in the switching valve and the momentary buildup of pressure across the valve during the switching process and consequent surge in flow when the sample loop is switched into the stream. Such injection valves normally employ sliding or rotating seals in contact with the sample which result in carry-over of previous samples trapped in the seal. This results in analysis error. Further, the requirement for sliding motion across ports places certain material requirements on the valve sealing element and restricts material selection for reasons other than chemical inertness and static sealing ability. For example, PTFE polymer with its excellent chemical inertness and static sealing ability is not usually considered satisfactory for sliding seals because of its extrusion into connecting ports and subsequent shearing off of the extruded portion of the seal.
The syringe injection method of sample introduction has been modified in a variety of ways in attempts to eliminate or mitigate some of its shortcomings. For example, Special fittings and related equipment have been developed for use with syringe injection (see U.S. Pat. No. 3,346,486).
Other methods of sample introduction have also been developed. For example, in the case of micro-scale operations, the packed micro-chromatographic columns have been plugged at the end with porous plugs and samples have been imbibed into one of the porous plugs followed by solvent development through such sample loaded plug (see U.S. Pat. No. 3,692,669)
Another sample preparation procedure involves depositing drops of liquid on filter paper and some rather complicated pieces of equipment have been devised to carry out such procedures in a way that creates test specimens of acceptable quality (see U.S. Pat. No. 3,266,554)
Notwithstanding all of such prior activity and developments relative to sample introduction in chromatographic analysis procedures, additional improvements in such art are sorely needed, particularly for use in micro-scale operations in order to obtain optimum chromatographic efficiency (minimum peak width).